This invention relates to methods for removing coatings from metal substrates. More specifically, it relates to the removal of coatings from the area within and around holes in the substrate.
Metal alloy components which are exposed to high temperatures (e.g., about 1000.degree. C.-1100.degree. C.) need to be protected from the potentially damaging effects of such an environment. Turbine engine components are but one example. Various techniques have been devised to maintain the temperature of the engine components below critical levels. One approach calls for the incorporation of internal cooling channels or "passage holes" in the component, through which cool air is forced during operation of the engine. For example, the holes may extend from the cooler surface of an airfoil to a "hot" surface which is exposed to the elevated temperatures. Cooling air (usually provided by the engine's compressor) is fed through the holes from the cooler surface to the hot surface. As long as the holes remain clear, the rushing air will assist in lowering the temperature of the hot metal surface and preventing melting or other degradation of the component.
The high temperature components are also frequently covered by specially-formulated protective coatings. These coatings, often referred to as thermal barrier coatings or "TBC's", effectively increase the operating temperatures of the alloys. Many of the protective coatings are ceramic-based, e.g., based on a material like zirconia (zirconium oxide). The coatings are frequently applied over an intervening bond layer, like those formed of aluminide, platinum aluminide, MCrAlY (where "M" is usually iron, nickel, or cobalt), or similar types of metallic coatings. When the TBC and the bond layer are heat-treated, a thin interlayer of alumina is formed between the substrate and the bond layer. (The TBC and the bond layer are sometimes collectively referred to herein as the "TBC system".)
When a protective coating on an engine component becomes worn or damaged, it must be carefully repaired, since direct exposure of the underlying substrate to excessive temperature may eventually cause the component to fail and damage other parts of the engine. The periodic overhaul of the TBC system usually involves complete removal of the TBC, followed by re-coating with fresh TBC. If the bond layer has been damaged, it is also removed and replaced. However, repeated removal of the bond layer is usually not permissible because of detrimental effects to the component, e.g., thinning of the component walls.
Various methods have been used to remove the TBC layer for eventual re-coating. Examples include grit blasting, water-jet treatment and caustic autoclave treatment. While such techniques are often effective for removing the TBC, there are sometimes problems associated with their use. For example, these techniques sometimes fail to remove the TBC from the passage holes. Moreover, even if the TBC is removed from the holes, the aggressive character of these removal techniques can damage the bond layer. They can also damage the substrate, "eating" into its thickness and thereby changing critical dimensions of the component. There are other drawbacks to some of the techniques. For example, grit-blasting is a labor-intensive and time-consuming process which detracts from the efficiency of TBC overhaul.
In U.S. Pat. No. 5,643,474 of D. Sangeeta, a wet chemical process for removing TBC coatings from the surface of a metal alloy component is described. The process involves treating the coated surface in an autoclave with an organic caustic solution. Temperature, pressure and time conditions are specifically maintained to completely remove the TBC without damaging the underlying bond layer or substrate surface.
The process in the Sangeeta patent is very effective for removing TBC material from flat and contoured surfaces. However, it is sometimes not as effective at removing the TBC material from the inner surface of cooling passage holes in the component, or from the component surface immediately adjacent the hole, i.e., at the "rim" of the hole. The interlayer of alumina between the TBC and the bond layer may be removed, but residual TBC material often remains in the hole. This TBC residue, which may adhere to the sides of the hole because of compressive stresses in the TBC, must be removed by an additional step, e.g., a mechanical technique which loosens the residue. However, the additional step may detract from the overall efficiency of the process.
Thus, new techniques for completely removing any type of a coating, such as (but not limited to) a TBC, from holes would be a useful addition to this area of technology. The techniques should be efficient and not labor-intensive. They should also preserve the integrity of the metal alloy surface and any bond layer remaining on the surface during the coating repair stage.